Acute myocardial infarction is the leading cause of death in most industrialized nations. The estimated annual incidence in the US is 865,000 events, with ST-segment elevation myocardial infarction (STEMI, severe AMI) comprising an estimated 500,000 events per year. Fibrinolytic therapy is widely utilized to restore coronary blood flow due to its widespread availability to the broad cross-section of patients. The current regimen, including tissue plasminogen activator, aspirin, clopidogrel and heparin, still induces inadequate coronary reperfusion in 30-40% of patients and early thrombotic reocclusion in 5-10% patients. Moreover, successful recanalization causes detrimental reperfusion injury that accounts for up to 50% of the final size of a myocardial infarct. Net clinical adverse outcomes remain 10-12% at 30 days after treatment and 16-17% STEMI patients die during 1-year follow-up. Moreover, both morbidity and mortality are dramatically increased in patients experiencing peri-procedure bleeding. Importantly, most of the recurrent MI and major bleeding events occur in the first hours and days after treatment. Consequently, the search for more efficacious and safer acute antithrombotic agents with effective attenuation of reperfusion injury remains the holy grail' of drug development. APT102 is an optimized human apyrase with two amino acid substitution that has significantly higher activity than the wild-type apyrases. This enzyme inhibits platelet activation and limits vascular inflammation by enzymatically hydrolyzing extracellular ADP and ATP. Adenosine is further generated by CD73 which prevents tissue damage during hypoxia and in the setting of cardiac reperfusion injury. In addition, APT102 prevents platelet desensitization and maintains homeostasis. With the Phase I award, we demonstrated that in the r-tPA induced fibrinolysis model in dogs, treatment of APT102 completely prevented re-occlusion, maintained normal blood flow, and profoundly reduced infarct size of hearts by 80% compared with clopidogrel. Meanwhile, APT102 did not prolong bleeding time, as did clopidogrel. The goal of this Phase II SBIR grant application is to rigorously determine whether in the coronary fibrinolysis model in dogs, APT102 promotes improved myocardial perfusion and left ventricular function with minimal bleeding compared with clopidogrel. The long-term goal is to develop APT102 as a highly effective antithrombotic and anti-inflammatory therapy with minimal bleeding risk that will profoundly improve efficacy and safety of acute treatment for AMI and other thrombotic patients.